The preoperative selection of intraocular lenses in the treatment of cataracts constitutes an important application. The most significant measured value is the axial length of the eye from the front of the cornea to the retina. According to the prior art, this is often measured contactlessly by optical interferometric methods, which are known under the name of PCI (partial coherence interferometry) or OCT (optical coherence tomography). In these methods structural transitions are illustrated as one-dimensional depth profiles (A-scan), as two-dimensional depth cut images (B-scan) or three-dimensional tomograms, wherein specular reflections on the optical interfaces, and/or light which is scattered in the different media of the eye, are detected.
In both measuring methods it is important that the measurement takes place along an axially oriented axis which corresponds to the visual axis. Otherwise errors can occur during the selection of the IOL which lead to a considerable defective vision of the patient after the implantation of the IOL.
In order to ensure the measurement along the visual axis with great reliability, during the measurement according to the prior art using the optical measuring instrument the patient is provided with a fixation light onto which the patient fixates his eye. In this way the visual axis of the eye is aligned with the main measuring axis of the device which simultaneously corresponds to the Z axis of the coordinate system of the measuring instrument. This can be seen from the literature [1]. If the device axis is aligned with the visual axis, the cornea and the retina are in most cases positioned sufficiently perpendicular to the main measuring axis, so that the measuring beams reflected from the cornea and the retina can be easily detected by the measuring instrument.
According to a first method described in the literature [2], the measurement of the axial length takes place by application of partial coherence interferometry in the double-beam method. In this case two beams with different optical path lengths impinge on the eye and are specularly reflected on the front of the cornea and the retina, resulting in interference. The signals at different optical path lengths are indicative of the eye length. Since a usable signal is only generated when a specular reflection from both the cornea and the retina is present, this method offers the advantage that for generation of a distance signal the cornea and the retina are approximately perpendicular to the measuring beam and thus to the device axis.
It has been shown experimentally that under these measurement conditions, which lead to a usable distance signal, in good approximation the device axis/measuring axis is identical to the visual axis, and the distance measured along the device axis corresponds to the axial length which is determinative for calculating the IOL.
Thus this measurement method virtually ensures that, in the case of excessively great deviations of the visual axis from the device axis, no incorrect measured value for the eye length is obtained and used for calculating the IOL.
However, it is a disadvantage that for the duration of the measurement time the patient must give a minimum amount of cooperation for the fixation. If this is not the case, no or very few, and thus statistically hardly reliable, measurements can be determined for the axial eye lengths.
It is a further disadvantage that measured values for B-scans or the measurement of the anterior chamber depth are difficult to realize. This is because during such measurements the tilted position of the measuring beam with respect to the boundary layers of either the cornea or the lens the boundary layers show no specular reflection which can also be detected by the device. Thus, newer methods which promise increased reliability in the selection of the intraocular lenses and require the measurement of the anterior chamber depth, lens thickness, or lens radii are not possible or only possible with difficulty.
According to a second method described in the literature [3], the measurement of intraocular distances takes place with the aid of one or more so-called B-scans which are obtained by means of optical coherence tomography. Thus the front face of the cornea and the retina as well as further tissue structures can be resolved. For example, cornea thickness, anterior chamber depth, and/or lens thickness can be determined.
The basic principle of the OCT method described for example in U.S. Pat. No. 5,321,501 A is based on white light interferometry and compares the propagation time of a signal with the aid of an interferometer (generally a Michelson or Mach-Zehnder interferometer). The arm with a known optical path length is used as a reference external to the object for the measurement arm. The interference of the signals from both arms produces a pattern from which the relative optical path length within an A-scan (single depth signal) can be read out. In the one-dimensional scanning grid methods the beam is then guided transversely in one or two directions, so that a two-dimensional B-scan or a three-dimensional tomogram can be recorded. Even in the B-scan this produces sufficient signals because with this method both specular reflections and also scattering in the object are detected.
However, in contrast to the double-beam method, in such methods it is not ensured by the measurement principle itself that the axis length (the axial length of the eye), which is important for calculating the intraocular lenses, is measured along the correct axis (the visual axis). This is because a recording and a signal are possible even if the measuring beam does not impinge perpendicularly on the front face of the cornea and/or is not aligned along the visual axis. The measurement along the device axis then supplies an A-scan which, considered alone, shows no discernible defect, even if it is not measured along the visual axis due to lack of fixation. However, reading off the axis length from the measurement along the device axis would generally lead to incorrect and systematically shortened measurement values, since in the event of lack of alignment of the measuring instrument with the visual axis, because of eye movement and/or lack of fixation, the A-scan measures laterally too far off the visual axis, which in a typically convex eye leads to a shortening of the cornea-retina distance.
With these B-scans the problem generally arises of the lateral attribution of the B-scans in terms of the eye. In this case, because of inaccurate alignment, eye movements not only during the measurement but also during the alignment of the measurement device with the eye lead to defective measurements.
If such eye movement is not taken into consideration, a B-scan and the intraocular distances which can be evaluated in such a B-scan are laterally offset in terms of the eye and thus incorrectly attributed. As a result it is not ensured that the A-scan along the device axis and/or the A-scan within a B-scan which runs along the device axis actually measures the eye length. Moreover, even with exact alignment, only a few A-scans—namely only those along the device axis—can be used for calculating the axial length, so that the measured axial length is encumbered by a relatively high statistical uncertainty.
A further method for determination of the distances between localized interfaces in the eye is already known from DE 10 2010 051 281 A1. With the aid of the scans which are recorded under different conditions and include at least two of the interfaces present in the eye, a parametric eye model can be appropriately adapted by a control and evaluation unit so that model-based determination of the eye biometrics can take place.
However, even with this solution it is problematic that the automatic evaluation of A-scans and B-scans to obtain biometric data is subject to a plurality of measuring situations and disruptions. These include, for example, attenuation of the measuring beam in the case of cataracts or defocusing of the measuring beam in the event of refractive errors or also the presence of pathological conditions.
In the as yet unpublished DE 10 2012 016 379.7 a method for measuring the axial length of an eye by means of OCT is described, wherein the alignment of the measuring instrument with the eye is monitored during the measurement. For this purpose the A-scans of the OCT with respect to the topography of the cornea are registered and the axial length of the eye is determined from the A-scan which is located at least approximately on the visual axis. Although in this way a reliable determination of the axial length is ensured, this always necessitates the topography of the cornea which is either already present or must first be measured.
A further solution which monitors and/or corrects the alignment of a measuring instrument with an eye is described for example in U.S. Pat. No. 7,452,077 B2. In this case the pattern of crossed B-scans is used to improve the misalignment, as the offset for each pair of B-scans relative to the vertex of the cornea is determined and if necessary corrected. It is a disadvantage here that the position of the visual axis on the cornea is not generally known.